1. Field of the Invention
The present invention relates to an integrated process for the concurrent production of methanol and synthetic natural gas from a carbonaceous solid or liquid material.
2. Description of the Prior Art
The prior art considered in the preparation of this specification is as follows: U.S. Pat. Nos. 1,735,925; 1,741,307; 1,741,308; 1,824,896; 1,831,179; 2,276,343; 3,598,527; and 3,666,682. All of these publications are to be considered as incorporated, in toto, herein by reference.
The present "energy crisis" is placing an increasing burden upon available fuel sources. Methanol and synthetic natural gas are fuels which can help alleviate this crisis. Synthetic natural gas, which is substantially methane, can be prepared from the gasification of a carbonaceous solid, e.g. coal, or liquid materials such as high boiling petroleum residues. However, such gasification processes are relatively expensive. Normally, methanol is manufactured by the catalytic conversion of a synthesis gas mixture containing carbon oxides and hydrogen at elevated pressure. The synthesis gas mixture is usually prepared by steam reforming a natural gas or a refinery gas stream having a high methane content. However, the synthesis gas mixture thus produced is too rich in hydrogen for the stoichiometry of the methanol synthesis reaction. This can be remedied by adding extraneous carbon dioxide to the methanol synthesis to achieve a suitable hydrogen/carbon oxides balance. Since methanol is manufactured at elevated pressures and carbon dioxide is usually available at lower pressures, it is usually necessary to compress the carbon dioxide, thus requiring an additional expenditure for the compressor.
The synthesis of methanol from natural gas is becoming increasingly unattractive as the availability of natural gas decreases and as the gas supplies are supplemented by synthetic substitutes derived from other fossil fuel sources; i.e., naphtha or heavy carbonaceous materials such as coal, coke, and petroleum residues. A synthesis gas suitable for use in the production of methanol from such heavy carbonaceous materials can be obtained by high temperature partial oxidation using essentially pure oxygen. However, this process is very expensive on the scale required for reasonably sized methanol plants.
In addition, the synthesis of methanol from carbon oxides and hydrogen is an equilibrium reaction which requires extensive recycle of unconverted reactants through a high pressure reactor to achieve the high degree of conversion necessary for an economical operation as well as the complete utilization of the expensive synthesis gas constituents. However, the accumulation of inerts in the recycle stream and an imbalance in the ratio of hydrogen to carbon oxides results in undesirable high purge rates of synthesis gas and requires expensive recycle compressor and reactor requirements. Furthermore, the synthesis of methanol from carbon oxides is highly exothermic but requires less hydrogen and releases less heat of reaction than does the production of an equivalent amount of methane. Since the shift reaction, which forms hydrogen from carbon monoxides and steam, is also strongly exothermic, the net result is that for a given amount of carbon oxide/hydrogen mixture, methanol production for fuel results in more Btu's being available to the consumer than from methane production. Similarly, the conversion of a given amount of synthesis gas mixture (CO, CO.sub.2 and H.sub.2) to methanol releases less waste heat in the process than does the conversion to methane.
Presently, it is considered uneconomical to build and operate low capacity methanol plants using reciprocating compressors for feed compression and recycle gas compression. In addition, as mentioned above, compressors are also frequently required in methanol synthesis because the steam reformed gas does not contain sufficient carbon oxides for the amount of methanol that could be prepared from a given amount of gas feed. Therefore, a methanol synthesis facility is often dependent upon the continued availability of an extraneous source of carbon dioxide, such as from a neighboring ammonia or hydrogen manufacturing facility. Thus, when the operation of a neighboring facility is interrupted such that the supply of carbon dioxide is adversely affected, methanol production may have to be reduced or even discontinued. However, these and other disadvantages commonly associated with the high costs of both synthetic natural gas production and methanol synthesis have now been overcome by the use of the present invention.